Thamó-Bozsó, Edit

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Thamó-Bozsó Edit

University of Szeged

136 Acta Mineralogica-Petrographica, Abstract Series, Szeged, Vol. 7, 2012 Joint 5th Mineral Sciences in the Carpathians Conference and 3rd Central-European Mineralogical Conference 20–21 April, 2012, University of Miskolc, Miskolc, Hungary LUMINESCENCE CHARACTERISTICS OF QUARTZ SEPARATED FROM LATE PLEISTOCENE-HOLOCENE SEDIMENTS OF THE CARPATHIAN BASIN  THAMÓ-BOZSÓ, E. Department of Geological Research, Geological Institute of Hungary, Stefánia út 14, H-1143 Budapest, Hungary E-mail: bozso@mafi.hu  Quartz is a frequently used mineral in the lumines-cence age dating, first of all in the optically stimulated luminescence (OSL) dating method. OSL age of the sediment gives the date of its last exposure to sunlight. After deposition and burial the ionizing radiation of the surrounding sediments and the cosmic radiation gener-ate free electrons, some of which become trapped in impurities and structural defects within quartz grains, and they store the absorbed energy. Initially the number of the trapped electrons is proportional to the dose of absorbed ionizing radiation. During heating or light exposure, the excited electrons escape from the traps, and the stored energy is released as luminescence light, TL or OSL signal respectively. There is a linear rela-tionship between the absorbed dose and the lumines-cence signal for low doses. However, in the period of burial, the traps gradually become saturated, and the further ionizing radiation does not cause growth in the luminescence, limiting the age dating usually about 100 ka at the case of quartz. The higher values of the environmental dose rate cause earlier saturation. Usu-ally only a small percentage of grains emit lumines-cence light, and the weak OSL signals give rise to prob-lems in dating. Late Pleistocene and Holocene sediments from dif-ferent parts of the Carpathian Basin were studied, e.g. fluvial sediments near the Danube, Tisza, Körös, Lápos and Ér rivers, from the Transdanubian Hills, and South Transdanubia; eolian dune sands from the Transdanu-bian Central Range, and Transtisza Region; loess and loess-like sediments from Transdanubia, and valley of Ér River. Most of the fluvial sediments are medium or fine sands, and there are only a few sandy clay and gravel samples among them. Samples were collected by opaque PVC tubes. Sample preparation was done under subdued red light in dark room. Quartz was extracted mainly from within the grain size fraction 90–160, or 100–200 µm using H2O2 to remove the organic material, and 10% HCl to dissolve carbonates. Aqueous solution of sodium polytungstate was used for density separation of the quartz-rich fraction, which was then etched with 40% HF for 60 or 90 min to remove any remaining feldspars and the outer layer from the quartz grains (AITKEN, 1998). The clean quartz grains were mounted on stainless-steel discs in an 8 mm diameter (large aliquot) monolayer using silicone spray. OSL measurements were made using a Riso TL/OSL DA-15C/D reader with a calibrated 90Sr/90Y beta source. Blue light-emitting diodes (LEDs) were used for the optical stimulation of quartz for 40 s, at 125 °C. Preheat temperature was 220, 240, or 260 °C according to the results of preheat plateau tests. A single-aliquot regen-erative-dose (SAR) protocol (WINTLE & MURRAY, 2006) was applied during the measurements. Environ-mental dose rates were calculated based on laboratory high-resolution gamma spectrometry analyses (Canberra GC3020, Eötvös Loránd Geophysical Institute of Hun-gary) of about 0.7–1 kg bulk samples from the sedi-ments surrounding the OSL samples. To characterise the luminescence of the separated quartz fractions their growth curves, saturation doses, and luminescence in-tensity were compared. Growth curves show the growth of luminescence re-sponse to increased doses of ionizing radiation up to 240 Gy in this study. Samples with fast-growing OSL reach the saturation level earlier, at about 80–100 Gy, e.g. some sediments from Transdanubia. Meanwhile samples with slowly growing luminescence become saturated later, above 200 or 240 Gy, e.g. fluvial sedi-ments of Lápos, Tisza, Danube, Körös, and Ér rivers.  Comparing the luminescence intensity, quartz frac-tions separated from some sediment of Ér Valley, Transdanubia, and near Tisza River exhibit the brightest OSL signals. But sediments of the Danube which were collected in the Vienna Basin have very dim OSL signal with consequent difficulties in age estimation. Some sands in the Transdanubian Central Range also show quite dim luminescence. Environmental dose rates of the studied sediments range between 0.7 and 3.8 Gy/ka. Due to the high radioactive Th, U, and K content the sediments of the Lápos River and loess samples from different areas have higher dose rates (above 2 Gy/ka) than the sands do. The research was supported by the Hungarian Na-tional Research Fund OTKA K-75801.   References AITKEN, M.J. (1998): An introduction to optical dat-ing. The dating of Quaternary sediments by the use of photon-stimulated luminescence. Oxford Univer-sity Press, Oxford, etc., p. 280. WINTLE, A.G. & MURRAY, A.S. (2006): Radiation Measurements, 41: 369–391.    

Luminescence characteristics of quartz separated from late pleistocene-holocene sediments of the Carpathian basin

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Luminescence characteristics of quartz separated from late pleistocene-holocene sediments of the Carpathian basin

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